A variety of drive systems for AC machines utilizing electronic switching to control the power applied to the machines are presently available commercially. These AC machine drives allow the torque and/or speed of the machine to be controlled to meet various requirements. Such machine drives typically require mechanical shaft transducers to provide feedback of shaft position and/or velocity. Feedback is generally required both for torque control (i.e., field orientation or vector control) and trajectory tracking, especially for control at zero and low speeds. However, shaft transducers and the associated wiring to provide the signals from the shaft transducers to the electronic drive add significantly to the cost and rate of failure of the system, and also add to the total volume and mass of the machine at the work site. Because induction machines are generally lower in cost and more rugged than other machine types, to a large extent the advantages of induction machines are the most compromised by the addition of such transducers.
Consequently, the desirability of eliminating position or velocity transducers in motor motion control applications has long been recognized. Several approaches have been proposed to allow estimation of the rotor position or velocity. However, if only torque control (and/or moderate accuracy speed control) is required by an application, knowledge of the magnetic flux vector location (and/or amplitude) is sufficient. Conventional methods of flux estimation that do not rely upon measured shaft position or velocity feedback fail at zero and low speeds.